1. Field of the Invention
The present invention relates to a sheet conveying roller to be used for conveying a sheet, for example, in a laser printer or the like.
2. Description of the Related Art
Various types of sheet conveying rollers are incorporated in a sheet conveying mechanism provided, for example, in an image forming apparatus such as a laser printer, an electrostatic copying machine, a plain paper facsimile machine, a copier-printer-facsimile multifunction machine or an inkjet printer, or machinery such as an automatic teller machine (ATM).
Examples of the sheet conveying rollers include a sheet feed roller, a transport roller, a platen roller and a sheet output roller, which are each adapted to be rotated in frictional contact with a sheet (the term “sheet” is herein defined to include a paper sheet, a plastic film and the like, and this definition is effective in the following description) to convey the sheet.
Such a conventional sheet conveying roller typically includes a tubular single-layer roller body made of an elastic material such as a crosslinking product of a rubber and having an outer peripheral surface serving as a contact surface to be brought into contact with a sheet, and a shaft inserted in a center through-hole of the roller body.
However, as the number of sheets conveyed by the sheet conveying roller is increased, the outer peripheral surface of the single-layer roller body of the sheet conveying roller is more liable to have a reduced friction coefficient with respect to a sheet. Problematically, this may result in sheet conveying failure and so-called squeal which is caused when a sheet slips on the outer peripheral surface.
To eliminate these problems, Japanese Patent No. 4593445 discloses a roller body which has a double-layer structure including a tubular non-porous inner layer and a tubular non-porous outer layer, the inner layer being made of a crosslinking product of a butyl rubber (for example, isobutylene-isoprene rubber: IIR) and having a Type-A Durometer hardness (JIS-A hardness) of not greater than 10, the outer layer being made of a crosslinking product of an ethylene propylene rubber, a silicone rubber or a urethane rubber and having a Type-A Durometer hardness of 25 to 60.
That is, the inner layer, which is a softer layer as described above, permits deformation of the outer layer to provide a sufficient contact area between the outer peripheral surface of the roller body and a sheet while suppressing the reduction in friction coefficient. The outer layer, which is harder than the inner layer, ensures proper balance between the abrasion resistance and the friction coefficient of the roller body. In addition, IIR is excellent in vibration damping property. This prevents the sheet conveying failure and the squeal for a longer period of time from the initial stage of use as compared with a case in which the roller body has the conventional single-layer structure.
For simplification of the structure of the sheet conveying roller and the production process for the sheet conveying roller, the inner layer and the outer layer are generally unified with each other by inserting the inner layer directly into the tubular outer layer without the intervention of an adhesive agent.
In recent years, there is a demand for using a thermoplastic elastomer as a base polymer for formation of the roller body of the sheet conveying roller. The thermoplastic elastomer is thermoplastic and hence easy to recycle.
To meet the demand, it is contemplated to form the outer layer of the roller body of the double-layer structure from a composition containing a urethane thermoplastic elastomer.
In this case, however, the inner layer inserted directly in the outer layer without the intervention of the adhesive agent is liable to slip-rotate with respect to the outer layer, thereby making it impossible to convey a sheet.
One conceivable cause of this problem is that the outer layer made of the urethane thermoplastic elastomer is liable to have lower stretchability than the conventional outer layer made of the crosslinking product of the urethane rubber or other rubber and have insufficient adhesiveness to the inserted inner layer.
More specifically, a great amount of oil should be added to the material for the inner layer in order to impart the inner layer with a Type-A Durometer hardness of not greater than 10. This results in bleeding of excess oil from the inner layer to reduce the adhesiveness.
Where the outer layer is made of the crosslinking product of the urethane rubber or other rubber as described in Japanese Patent No. 4593445, the outer layer is kept in intimate contact with the inner layer by sufficient stretchability thereof and, therefore, is substantially free from slip-rotation with respect to the inner layer.
However, the outer layer made of the less stretchable urethane thermoplastic elastomer has lower adhesiveness with respect to the inner layer and, therefore, is more likely to suffer from slip-rotation which may be caused by the influence of the oil bleeding out of the inner layer.
It is an object of the present invention to provide a sheet conveying roller which has a double-layer structure including a tubular outer layer and an inner layer inserted directly in the tubular outer layer without the intervention of an adhesive agent, and is excellent in various characteristic properties because of its double-layer structure and substantially free from slip-rotation between the inner layer and the outer layer though the outer layer is made of a urethane thermoplastic elastomer.